configure: Don't special case Cygwin to use gnu99, define _XOPEN_SOURCE instead

configure: Don't special case Cygwin to use gnu99, define _XOPEN_SOURCE instead

Revert "build: Build on Cygwin with gnu99 instead of c99." and define
_XOPEN_SOURCE appropriately.

This reverts commit 53e36d333c9b619c1a5fe9a8d2d08665654b0234.

Since Cygwin 1.7.18 (April 2013), it's headers correctly prototype strtoll()
when using -std=c99, and correctly prototype strdup() when _XOPEN_SOURCE is
defined appropriately, so this workaround is no longer needed.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Cc: Vinson Lee <vlee@freedesktop.org>